Hi

There's unfortunately no simple solution. Check out the following page and read the comments.

http://captnswing.net/howto/itunes/

I recommend to create a backup of the entire library and to resore the original iTunes settings (move the iTunes directory back to /Users/username/Music) otherwise you'll have problems to follow the instructions.

Stief's suggestion along with DAAP would also work but it is probably not that what you're looking for. With iTunes 4.7 and Fast User Switching it's also possible to share music between multiple accounts (to destroy your hopes: it's based on DAAP aka iTunes Music Sharing -- previous iTunes version refused to launch if one instance of iTunes was already open).